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2. INVESTIGATING THE COPPER CONTENT OF BRASS / Student
Handout
2. INVESTIGATING THE COPPER CONTENT OF BRASS / Student
Handout
2. INVESTIGATING THE COPPER CONTENT OF BRASS
Introduction
Brass is an alloy of various metals including copper, zinc,
aluminum and tin. Copper, zinc and iron are typically the most
abundant. The relative proportions of these metals greatly
influence the properties and utility of brass. Such properties as
color, hardness, ductility and conductivity change depending on the
content of the various metals in the alloy.
Concepts
Electromagnetic spectrum
Spectroscopy or colorimetry
Beer’s law
Calibration curve
Solution concentration
Electron transitions
Background
Alloys are solid substances that are mixtures of two or more
solid elements. They are classified as either substitutional or
interstitial depending on the atomic arrangement that forms the
alloy. The components of an alloy affect many of the physical
properties of an alloy such as hardness or strength. Steel is an
interstitial alloy that combines elemental iron with non-metallic
carbon or silicon. It is considered interstitial alloy because the
very small carbon atoms fit into interstices of the iron matrix.
The increase in hardness and tensile strength of steel makes it a
much more attractive metal for use in construction compared to
iron.
Brass, on the other hand, is a substitutional alloy that is a
combination of copper and a number of other metals like zinc,
aluminum, tin, lead or iron in varying quantities. In a
substitutional alloy, the sizes of component atoms is comparable
such that one can be substituted for the other. The percent copper
content of any given brass alloy affects its mechanical strength,
ductility, hardness, electrical conductivity and resistance to
corrosion. For example, the ratio of copper to tin in brass
dictates its uses ranging from musical instruments, construction,
decorative elements and electrical switches. The table below shows
a few of the 300 known formulations of brass and their common
uses.
Type of Brass
% Cu
% Zn
Use and Other Information
High brass
65
35
Used in springs, rivets and screws
Low brass
80
20
Used in flexible metal hoses, high ductility
Manganese brass
70
29
Used in “gold” dollar coins in U.S.
Naval brass
59
40
Used in marine construction; resists corrosion
Nordic gold
89.5
5
Used in “gold” Euro coins
Yellow brass
67
33
Decorative elements, saxophones, trombones
Spectroscopy is the study of the interaction of electromagnetic
radiation and matter. Visible spectroscopy is a simple tool to
determine the percent copper content in a sample of brass because
the absorption of electromagnetic radiation causes different kinds
of transitions within the substance. The energies of these
transitions are characteristic of a specific atom or molecule. The
concentration of a colored transition metal ion solution can also
be determined by measuring the color intensity.
In this investigation, a brass sample is dissolved in nitric
acid and only one colored species is produced in solution. The
copper (II) ion has a blue color when in solution. The
concentration of copper in the brass sample is measured by
evaluating the intensity of the blue color of the solution. The
absorbance of the colored solution is directly proportional to its
concentration
A = abc
where A is absorbance, a is the molar absorptivity with units of
L mol-1 cm-1, b is the path length measured in cm of the cuvette in
which the sample is contained, and c is the concentration of the
compound in solution expressed in mol L-1. A Beer’s law plot can be
constructed using solutions of known concentration. Plotting
absorbance versus known concentrations for a series of standard
solutions will generate a linear calibration curve
y = mx + b
where y = absorbance, x = concentration and b = y-intercept. The
copper ion concentration of the brass sample is then determined
using this plot.
Pre-Lab Questions
In this lab, you will be dissolving copper-containing brass in a
concentrated oxidizing acid, nitric acid. A gas, nitrogen dioxide,
is a by-product of the oxidative process that also produces aqueous
copper(II) nitrate and liquid water. Write the complete balanced
equation for this reaction.
Write the complete and net ionic equations for this
reaction.
If a sample of brass is 80% copper by mass, what is the minimum
volume of 6 M nitric acid needed to react completely with a 1.00 g
sample of brass?
Copper(II) ions are transmitted as the color blue in solution.
Based on the color wheel and principle of complementary colors,
which colors or wavelengths of light would you expect to be the
most strongly absorbed by Cu2+ ions?
Materials and Equipment
Use the following materials to complete the initial
investigation. For conducting an experiment of your own design,
check with your teacher to see what materials and equipment are
available.
Data collection system
Distilled water in wash bottle
Wireless colorimeter
0.2 M Copper(II) nitrate solution
Plastic cuvettes
Volumetric flask, 100-mL
Plastic Beral pipettes or eyedroppers
Brass samples, 1 g each
Test tubes and racks
150-mL beakers
Precision balance (to 0.001 g)
10-mL graduated cylinders
50-mL graduated cylinders
Concentrated nitric acid (HNO3)
Transition metal solutions: 0.1 M Copper(II) nitrate, copper(II)
sulfate, iron(III) nitrate, iron(II) sulfate1, zinc nitrate1
Watch glasses
Safety
Follow these important safety precautions in addition to your
regular classroom procedures:
Wear safety goggles at all times
Concentrated nitric acid is severely corrosive and toxic by
ingestion or inhalation. Work must be performed in a fume hood or
well-ventilated area.
Nitrogen dioxide is a toxic, reddish gas and is a by-product of
this reaction.
Copper(II) sulfate and copper(II) nitrate solutions are skin
irritants and toxic by ingestion.
Iron(III) chloride, iron(III) nitrate, zinc nitrate and zinc
sulfate are also skin irritants and direct contact with skin and
body tissues must be avoided.
Wash hands thoroughly with soap and water before leaving
laboratory.
Dispose of solutions as directed by Material Safety Data
Sheet.
Disposal
If your drain system is connected to a sanitary sewer system,
the following instructions apply. Excess concentrated nitric acid
may be neutralized with sodium bicarbonate and flushed down the
drain with a minimum 20-fold excess of water. Copper(II) sulfate,
copper(II) nitrate, iron(III) chloride, iron(III) nitrate, zinc
nitrate and zinc sulfate solutions may be rinsed down the drain
with an excess of water. All 0.1 M spectroscopic solutions may also
be disposed of in the drain. If your drain system does not empty
into a wastewater facility, non-flammable and inorganic aqueous
waste must be disposed of in a landfill or via a licensed hazardous
waste provider.
Initial Investigation
Visible Spectra of Transition Metal Ions
1.Open the 02 Investigating the Copper Content of Brass lab file
in SPARKvue. Use the Bluetooth icon to connect the colorimeter to
the data collection system.
2.Calibrate the colorimeter using distilled water. Remember that
the cuvettes must be handled by the ribbed sides and use Kimwipes®
to clean the clear sides as needed. You should read zero for
absorbance and 100% for transmittance.
3.Follow directions for colorimetric measurements from your
instructor.
4.Your teacher will provide you with two samples of transition
metal solutions.
5.Transfer solutions of transition metals to cuvettes filling to
three-fourths full. Insert into the cuvette port of the colorimeter
and record the absorbance data.
6.Of the given wavelengths of light, determine the optimum
wavelength that demonstrates your sample’s maximal absorbance.
7.Report spectral data to the class for advanced investigation
activity.
8.Do Zn2+ ions absorb visible light? Discuss the answer in terms
of the physical appearance of Zn2+ solutions and the electronic
structure of Zn2+ ions.
9.If Fe3+ ions are present in your brass sample, do you expect
them to interfere with Cu2+ analysis at its optimal wavelength?
Explain your answer.
Zinc
Copper
Iron
Calibration Curve for Copper(II) Ion Solutions
1.Prepare a series of dilutions of stock 0.2 M Cu(NO3)2 as
specified in Table 1. Calculate the final concentration of each
sample.
2.Navigate to Page 2 of the lab file. Enter the concentration
for each sample in the Molarity column in SPARKvue. Set the second
column to the absorbance for the optimal wavelength for the Cu2+
ion (from Step 6 of the previous activity). Set the third column to
the transmittance for the optimal wavelength.
3.Measure and record the absorbance and transmittance of your
first dilution sample at the optimal wavelength. Press the green
“check” mark to record the reading. Record all absorbance and
transmittance data in Table 1.
4.Remove the cuvette from the colorimeter and replace with a
second sample. Repeat the necessary steps to test the remaining dye
dilution samples.
5.When completed, stop data collection and save your
experiment.
6.Navigate to Page 3 in SPARKvue. Choose the absorbance at the
optimal wavelength to complete the y-axis. Use SPARKvue’s graphing
tools to determine the linearity of your calibration curve.
Table 1: Calibration Curve of Cu2+ Ions
0.2 M Cu(NO3)2 Stock
Stock Solution
(mL)
Distilled Water
(mL)
Concentration2
(M)
Absorbance
(-log T)
%Transmittance
1
10
0
2
8
2
3
6
4
4
4
6
5
3
7
6
2
8
7
1
9
Advanced Investigation
Percent Copper in Brass
1.Weigh out approximately 1.0 g brass in a small plastic weigh
boat and record its mass to the 0.001 g.
2.In a chemical fume hood, transfer brass metal to a beaker.
Assuming your sample is 100% copper, add the appropriate volume of
concentrated 15.6 M HNO3 needed for a complete reaction. Show your
calculations. Cover with a watch glass and allow the reaction to
proceed for 10-20 minutes or until your brass sample has
dissolved.
3.Still working in the hood, add 30 mL distilled water to the
reaction mix in the beaker. Gently swirl or stir solution with a
glass rod and remove from the hood.
4.Transfer the solution to a 100 mL volumetric flask. Rinse the
reaction beaker with 5 mL distilled water then transfer contents to
the volumetric flask. Repeat the rinse two more times. Finally, add
enough water to the volumetric flask to fill to the 100 mL mark.
Cap the flask and mix well.
5.Using a plastic pipette, transfer a sample of the reaction
mixture to a cuvette and measure absorbance at the optimal
wavelength for Cu2+. Calculate the percent copper in the original
brass sample.
Hint: Track your dilutions of the original reaction product.
6.Dispose of all materials as instructed by your instructor.
Extended Inquiry Investigation
Following the procedures used above, similar investigations can
be conducted to investigate the copper content of other everyday
items. For example, the laboratory activity can be extended to
determine the amount of copper in a penny, a piece of wire or a
small brass bolt from the hardware store. The item must be
sufficiently small to facilitate the reaction with nitric acid.
Have your lab group design a procedure to determine the copper
content of your everyday item. As a team, you must determine if a
new calibration curve must be created or if the standard curve
developed in this lab's Initial Investigation will suffice for this
analysis. Once the percent copper of your everyday item has been
determined, compare your findings with theoretical values (online
or literature sources) and calculate the percent error from your
experimental determination.
Synthesis Questions
1.A solution of potassium permanganate (KMnO4) is purple.
Describe the absorption spectrum of KMnO4.
2.An absorption spectrum shows significant absorption in the
blue and little or no absorption in the green and red range. What
color do you think the solution is?
3.MnSO4 has a very faint yellow color. Which color of photons
are NOT absorbed?
AP® Chemistry Review Questions
1.A Beer’s law experiment is performed to determine the
concentration of an unknown solution of nickel(II) ion in solution.
The absorbances of 5 stock solutions of NiCl2 are collected at a
wavelength of 680 nm. The data is displayed below: Ni2+ ion has a
molar absorptivity of 0.005292 at 680 nm.
Concentration of NiCl2
(mM)
Absorbance
(nm)
20
0.11
60
0.32
125
0.67
150
0.80
200
1.06
a.Using the blank graph below, plot the absorbance data
including proper labeling of x- and y-axes.
b.The nickel(II) chloride solutions are green. Using the color
wheel in the Pre-Lab Questions, explain why the sample absorbs best
around 680 nm and not 500 nm.
c.Explain at the particle level, why the absorbance of a
solution increases as concentration increases.
d.Assuming the y-intercept is negligible, predict the absorbance
for a sample with a concentration of 163 mM.
e.Predict the concentration of a sample with an absorbance of
0.43.
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2pasco / ps-2828A
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